Recent studies have elucidated several genetic alterations that occur during the development of colorectal tumors, the most common of which are deletions of the short arm of chromosome 17 (17p) and the long arm of chromosome 18 (18q). Vogelstein, et al., Science, vol. 244, p. 207 (1989); Fearon, et al., Science, vol. 238, p. 193 (1987); Muleris, et al., Ann. Genet. (Paris), vol. 28, p. 206 (1985); Monpezat, et al., Int. J. Cancer, vol. 41, p. 404 (1988). While some genetic alterations such as RAS gene mutations appear to occur relatively early during colorectal tumor development, chromosome 18q deletions are often late events associated with the transition from Class II to Class III adenomas or the transition from the benign (adenomatous) to the malignant (carcinomatous) state, (Vogelstein et al., New England Journal of Medicine, Vol. 319, p. 525, 1988) and appear to be related to metastasis-and decreased survival time (Kern, et al., JAMA, vol. 261, pp. 13099-13103, 1989). Because carcinomas are often lethal, while the precursor adenomas are uniformly curable, the delineation of the molecular events mediating this transition are of considerable importance.
Allelic deletions have been reported to encompass a large area of chromosome 18q. (Vogelstein, et al., ibid.) This area is known to contain the BCL-2 gene (Tsujimoto, et al., Science, vol. 226, p. 1097 (1984); and Cleary, et al., Cell, vol. 47, p. 19 (1986),) the gastrin-releasing peptide gene (Spindel, et al., Proc. Natl. Acad. Sci., USA, vol. 81, p. 5699 (1984),) and the cellular homologue of the YES-1 oncogene (Semba, et al., Science, vol. 227, p. 1038 (1985) and Yoshida, et al., Cytogenet. Cell Genet., vol. 40, p. 786 (1985)). All of these genes are known to be associated with cancers. If a particular region of the chromosome is the target of the deletions, i.e., it is involved in the neoplastic process, precise delineation of the region is necessary to provide methods of diagnosis as well as therapy.
According to the model of Knudson for tumorigenesis (Cancer Research, vol. 45, p. 1482, 1985), there are tumor suppressor genes in all normal cells which, when they become non-functional due to mutation, cause neoplastic development. Evidence for this model has been found in the cases of retinoblastoma and colorectal tumors. The implicated suppressor genes in those tumors, RB and p53, were found to be deleted or altered in many cases of the tumors studied. There is a need in the art of cancer diagnosis and therapy to find other suppressor genes involved in tumorigenesis, so that defects in the suppressor genes or effected cells can be detected and the defects cured to abate or reverse the neoplastic processes.